Figure REF  is a dialog of completing a multi-domain task produced by a user and a dialog system [1]}, [2]}. The user starts the conversation by asking for a recommendation of a museum in the center of town. The system identifies the user request, and provides a recommendation based on the search result from an attraction DB. Then, the user wants to book a table in a restaurant in the same area.
We can see that through the conversation, the system develops belief states, which can be viewed as the system’s understanding of what the user needs and what is available in the DB. Based on belief state, the system picks the next action, either asking for clarification or providing the user with information being requested.
This example also presents some challenges in conversational search. For example, the agent needs to understand that the “same area” refers to “centre of town”, (i.e., the so-called co-reference resolution problem), then identifies a proper entity from the restaurant-booking DB to make the reservation.
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